There are a number of particular problems in dealing with emergency calls that do not arise for regular calls. For example, in order that emergency service vehicles or other assistance can be dispatched to the correct destination promptly, accurate information about the location of the caller is needed. Previously, in conventional switched telephone networks, it has been possible to provide the caller location information relatively easily because telephone handsets are typically fixed in particular locations. Static database entries can then be made in a database accessible to the emergency services associating for example, a subscribers' home address and telephone number. However, for mobile communication systems and also for nomadic systems use of such static database entries is not possible because the location of a communications terminal varies over time.
Another problem concerns routing emergency calls to the correct destination. For regular calls this is not such an issue because the caller enters specific details of the required call destination. However, for emergency calls a universal code is used such as 911 in North America and 112 in Europe. This universal code cannot be used to identify the destination of the call. Generally, an emergency call needs to be routed to a particular geographical answering point for servicing. This answering point is often referred to as a Public Safety Answering Point (PSAP). The jurisdiction for emergency services answering points is typically quite small, for example, at the county level in the USA. This information about the location of the caller is needed to determine which emergency services answering point to route the call to. Misrouting of calls to the wrong answering point leads to costs in transferring calls, impacts reliability, and leads to delays which are significant in life threatening situations. Previously, in conventional switched telephone networks, this location information was relatively easy to obtain because static database entries could be used as mentioned above. However, this is not possible for mobile and nomadic communications systems.
One proposal has been to update or refresh the database entries every 24 hours. However, this approach cannot cope with situations where a user terminal changes location more than once a day. Also, changes to the existing emergency services network infrastructure are required in order to enable the database to be updated daily.
More detail about how existing voice networks interface to the emergency services network is now given. The primary existing voice networks that do interface to emergency services are the PSTN (public switched telephone network) as served by LECs (local exchange carriers) and the various mobile networks operated by the cellular carriers.
The emergency services network, from this perspective, can be regarded as being made up of Selective Routers (SRs), Automatic Location Identification (ALI) databases, both local and national, and the Public Safety Answering Points (PSAPs) themselves with their various CAMA (centralized automatic message accounting), and other, trunk connections and various data connections for querying the ALIs. Of course, beyond these network elements are the public safety organisations themselves (Police, Fire, Ambulance) and the communications networks that support them.
The location of the subscriber, who is dialing emergency services, is used for two key purposes. The first is routing of the call, ultimately to the right PSAP, and the second is in the delivery of the location, for display, to the PSAP operator in order that emergency response units can be dispatched to the correct location.
In wireline voice networks, there is an association between the phone number of the subscriber (The Calling Line Identifier—CLID) and that subscriber's location. This is generally, the home address of the subscriber as maintained by their local exchange carrier. In this case, the CLID becomes a ready-reference to location.
Similarly, the incoming line to the local exchange switch and the switch itself provides an explicit indication of the appropriate routing of 911 calls. This permits the local exchange to work from a static configuration in terms of selecting the outgoing trunk on which to place the call so it goes to the correct selective router. The selective router, in turn, can use the same static association and CLID information to ensure that the call is routed to the correct serving PSAP for the subscriber's address.
In cellular systems, the association between the subscriber's location and their CLID is lost. Being, by definition, mobile a cellular subscriber can be anywhere within the wireless network's area of coverage. Similarly, there is no physical wired line corresponding to the source of the call from which to associate a route to the correct destination. In cellular networks, however, there is a physical serving cell from which the call is initiated. The geographic granularity of these cell locations is generally sufficiently fine for the mobile switch to determine the correct trunk route to a corresponding selective router. In many cases, this also provides sufficient accuracy for the selective router to determine which PSAP the caller should be connected with.
It is an internal procedure for the mobile switch to associate an outgoing trunk route with a serving cell. However, some signaling is required for an MSC (mobile switching center) to pass this same information along to the selective router so that it can determine the correct PSAP. The TR45 standard, J-STD-036 “Enhanced Wireless 9-1-1 Phase 2”, Telecommunications Industry Association, 2000, defines mechanisms for doing this. The routing information is passed to the selective router in the ISUP (ISDN user part) call setup signaling in one or other newly defined parameters called the Emergency Services Routing Digits (ESRD) or the Emergency Services Routing Key (ESRK). The selective router examines the value of the ESRD/ESRK parameter in the call setup signaling and routes the call to the correct PSAP based on this value.
Note that there are circumstances where cell boundaries can span the boundaries of PSAP catchment areas. In this case, and ESRD or ESRK determined from a serving cell may not provide a reliable indication of a route to the correct PSAP. Both ANSI-41 (generally TDMA, and CDMA) and 3GPP (generally GSM, EDGE, and UMTS) cellular networks have identified functionality to address this. In ANSI-41 networks a functional element known as a Coordinate Routing Database (CRDB) is defined. The network can consult the CRDB and, based on the geographic location of the caller (determined by different positioning technologies such as forward link trilateration, pilot strength measurements, time of arrival measurements, etc.), it will return an appropriate value of the routing parameter. As long as the geographic location is an improvement in accuracy over the cell location, this mitigates the problem of misrouted calls. Similarly 3GPP networks allow the switch to request a refined routing key value from the Gateway Mobile Location Center (GMLC) based on the geographic location of the caller.
The second, independent, area in which location comes into play in E911 calling is the display of the caller's location to the PSAP operator. The need for this is that the PSAP operator can facilitate more rapid despatch of the emergency service response units if the network can deliver the location rather than relying on getting this information from the caller—particularly where the caller may be unable to provide this information.
In a wireline voice network, necessary subscriber (or, at least, calling line) address information is stored in a database known as an Automatic Location Identification, or ALI, database. On receipt of an emergency call and, armed with the caller's CLID, the PSAP is able to query this database and receive, in return, the street address (also known as a civic address) information associated with the CLID. The physical interface over which the PSAP makes this query is variable. It may be an IP based interface over dial-up or broadband or it may be made over an X.25 packet interface. Similarly, the ALI may physically be co-located within the LEC and selective router, or it may be a remote national ALI handling the request directly or in tandem from the local ALI. Similarly, the protocol may vary but one known as PAM (PSAP to ALI message specification) is in common usage. These details are contained within the emergency network itself and not generally a concern of the larger voice network on the far side of the selective router.
In a cellular network, the same level of detachment with respect to this function is not possible. To begin with, the location of the caller is variable both initially and during the period of an emergency call. It is no longer possible to rely on a static database of location information that can provide an address against a CLID. It now becomes necessary for the PSAP to be able to request a dynamic location both for the initial position of the caller but also for any changes during the call. In addition, a civic address may no longer be pertinent to the location of the caller. By nature, cellular networks cover wide and varying types of territory. A conventional street address may no longer apply to a caller's location. Indeed, they may not even be in or by a street as the term is commonly understood. For this reason, a more universal reference system for location needs to be used. The solution generally adopted and, once more defined in J-STD-036 as referenced above, is to use geospatial co-ordinates—or latitude and longitude—as defined in the WGS-84 coordinate system (Military Standard WGS84 Metric MIL-STD-2401 (11 Jan. 1994): “Military Standard Department of Defence World Geodetic System (WGS)”).
While J-STD-036 does define mechanism whereby this geospatial location can be delivered in the ISUP call setup signaling, it can be generally acknowledged that PSAPs do not support the necessary signaling interfaces nor customer premises equipment to receive and display this information. Also, there is no mechanism whereby an updated location can be delivered in the ISUP signaling. For these reasons, J-STD-036 identifies a new interface that the emergency network can use to query the cellular network. This interface is assigned the identifier of E2 and both J-STD-036 and NENA “NENA Standard for the Implementation of the Wireless Emergency Service Protocol E2 Interface” define a protocol which can be used over this interface called the emergency services protocol.
On receipt of an emergency call arising from a cellular network, the PSAP can initiate, via the serving ALI, a request on the cellular network to provide the geodetic location of the caller. This request is made over the E2 interface in a message called the EPOSREQ (Emergency Position Request) with the response message identified as the esposreq. The location of the caller is determined by positioning capabilities native to the cellular network itself and different systems of network measurement, triangulation, or special handset capabilities such as GPS (Global Positioning System) are used.
As described above, the network mechanisms and procedures defined in JSTD-036 are around the provision of a geodetic (latitude and longitude) type location for the caller. This obviously implies a capability on the part of the PSAP to display location information of this type to the PSAP operator. There is also consideration supported in the E2 interface messaging that allows the delivery of civic address type information.
One application of this facility is in the support of PSAPs which are not equipped with the capability to receive and display geodetic type location information. This is part of what is often referred to as a Phase 1 E911 capability for cellular networks. Enhanced 911 calling was introduced in two phases into the cellular and emergency services networks. Phase 2 defined the capabilities for delivering, generally more accurate, geodetic location information from the network. Phase 1 was generally targeted at providing location information to the accuracy of a serving base station location but, perhaps more importantly, that location information is delivered to the PSAP as a more conventional street, or civic, address associated with that base station. Depending on the nature of the PSAP, the ALI may provide the geodetic position and/or the phase1 civic address type information in response to the location bid.
Just as cellular networks have specific characteristics that result in new considerations for E911 compared to conventional wireline voice networks, so too do IP based voice (VoIP) networks. VoIP network users have much in common with cellular network users in that there is no specific physical point of connection which dictates their identity. Just as a cellular phone can attach to the network anywhere that there is a point of coverage, so too can an IP based phone client attach to an IP network at many and varied points and take advantage of the voice service. From this perspective, it becomes necessary to view VoIP clients as essentially nomadic or even fully mobile to ensure that all usage scenarios are covered. For certain, many VoIP clients may be relatively static in terms of location (for example, a conventional form factor desktop phone with integrated VoIP client software will tend to be as stationary as any conventional wireline desktop phone) however, this situation is not explicitly predictable by the network, so an architecture that addresses mobility ensures that all usage scenarios are covered.
In terms of emergency call routing, the VoIP network introduces some additional challenges over wireline or cellular networks. In particular, the access network associated with a VoIP network can be highly distended. That is to say, in wireline the phone is tied to the specific local switch by the incoming line, in cellular the mobile switch has specific knowledge of the serving cell which has some degree of geographic association with that switch. But, in VoIP, the client may be attached to the network in another city, state, or, even, country than the one in which the serving call server is located. There is not an immediate association to location that the call server can use to directly determine a route to a selective router before, even, the correct PSAP can be selected.
Similarly, in terms of location delivery and display, a VoIP client may be appropriately identified by a street address, being on a relatively static access point, or it may be more appropriately identified against a geodetic location, as in the case of a VoIP client connected by a wide area broadband wireless network.